The present invention relates to a zoom lens barrel that contains an inner barrel nested in an outer barrel, and more specifically, to a structure for retaining the inner barrel within the outer barrel.
Zoom lens barrels consisting of one or more telescoping lens barrels is commonly in use in "compact" cameras. "Telescoping", in this context, does not refer to the optical properties of the lenses, but rather to a mechanical structure consisting of parts that fit and slide one within another, in the manner of the tubes of a jointed telescope. The telescoping zoom lens barrel, in order to move an inner barrel along a direction of the optical axis, has internal threads formed on an inner surface of the outer barrel and external threads formed on the outer surface of the inner barrel. The threads are engaged, end one of the inner or outer barrels is rotated relative to the other. This relationship can continue between multiple nested barrels.
For example, the external threads are formed in a narrow band on the camera-body side portion of the inner barrel, while the remainder of the outer surface is a smooth cylindrical surface having no projections. The engagement is between the externally threaded band and inner threads extending over the length of the outer barrel, enabling the smooth cylindrical portion of the inner barrel to extend beyond the outer barrel (telescoping). In some cases, a ring is attached inside the edge of the outer barrel to cover the gap between the outer barrel and the inner barrel. The ring member improves appearance and blocks light at barrel interconnections.
Ideally, the clearance between the last external threads and the ring member is minimized, when the zoom lens barrel is fully telescoped, to achieve the longest possible telescoping extension of the inner barrel. However, when the clearance is very small, or so small that contact between the ring and external thread is possible, detachment of the ring member may occur if the user inadvertently pulls the inner barrel while the zoom lens barrel is fully telescoped. More particularly, a triple-barrel telescoping zoom lens is more likely to suffer a detachment of the ring member than a double-barrel zoom lens, as a triple-barrel lens operates with a longer (i.e., larger) backlash than the double-barrel lens.